1. Field of the Invention
This invention relates to an electrical connector assembly employed to make electrical connections in a plurality of electrical signal lines in which the impedance of the signals transmitted through the connector varies insignificantly from the impedance of the interconnected signal lines and more particularly the electrical connector embodying this invention is intended for use in making a right angle interconnection between two orthogonal conductor arrays such as two printed circuit boards or a printed circuit board and a flat transmission cable.
2. Description of the Prior Art
U.S. Pat. No. 4,762,500 and U.S. Pat. No. 4,747,787 depict electrical connectors intended for the impedance matched electrical interconnection of a plurality of circuits, either on a pair of printed circuit boards or between a transmission cable and a printed circuit board. These connectors employ a central ground bus having a plurality of receptacle signal terminals located in parallel rows on opposite sides of the central ground bus. Both the receptacle terminals and the ground busses of mating connector halves are fully mateable so as to maintain the same impedance between the two connectors. Each connector generally employs microstrip transmission principles to establish an interconnection, thus minimizing changes in impedance between the incoming component, either a cable or a printed circuit board, and the outgoing component, again either a cable or the circuits on a printed circuit board. U.S. Pat. No. 4,695,106 and U.S. Pat. No. 4,762,500 in particular disclose electrical connectors for making a plurality of electrical connections between multiple signal lines on two orthogonally oriented printed circuit boards.
It has proved difficult to achieve the required impedance levels in a manufacturable product employing the components depicted in those patents. A better understanding of the elements of a truly manufacturable matched impedance connector has now been achieved. It has been found that a connector designed in accordance with established geometrical relationships used for microstrip printed circuit board design does not result in a microstrip connector design having acceptable performance. Unlike a microstrip circuit board, electrical connectors have no signal conductors or ground conductors in intimate contact with surrounding dielectric material. Air spaces between metal and plastic, resulting from tolerances or the need to accommodate thermal expansion differentials, affect the dielectric constant of housing materials to a different extent than air above a circuit board trace and the substrate below that trace. Furthermore dielectric material exists above the connector signal contacts while no dielectric is present in that location for microstrip circuit boards. In part, these differences account for some of the inadequacies of conventional microstrip formula when applied directly to an electrical connector employing microstrip principles.
It has been found however that by positioning the inner peripheral surface of receptacle signal terminals closer to an outer peripheral surface of a receptacle ground bus than the spacing between adjacent surfaces of signal terminals, the desired characteristic impedance can be achieved, especially if spacings between signal terminals and the receptacle housing ground bus are maintained substantially constant, at least for incremental lengths greater than the wavelength of signals to be transmitted. The preferred embodiment of the invention depicted herein comprises a practical and manufacturable electrical connector assembly which can be used to interconnect circuits having an impedance of 50 ohms and the connector depicted herein will remain substantially transparent, thus having little or no affect on the signals transmitted through the connector. In particular, the electrical connectors embodying this invention provide a means for maintaining the positional relationship between signal terminals and a parallel ground bus through a right angle without introducing significant differences in the impedance of signals transmitted through the electrical connector.